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Rifles, Reloading, Optics, Equipment
Reloading
measuring cases to .0001
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<blockquote data-quote="Bart B" data-source="post: 635410" data-attributes="member: 5302"><p>Whopper Stopper, I've found it's not necessary to resize bottleneck cases to the nth degree of uniformity. Especially when brand spanking new ones will shoot 1/2 MOA at 600 yards and 3/4 MOA at 1000. They'll have a case headspace spread of about 2/1000ths inch; sometimes more. With cases not being perfectly uniform in metalurgy properties as well as wall thickness, the best us humans can hope for is a 1/1000th inch spread in sized bottleneck case headspace from head to shoulder datum. But 2/1000ths inch is good enough for what's called head clearance; the difference between chamber headspace and case headspace. Remember when a rimless bottleneck (as well as a properly sized belted bottleneck) round fires, the case shoulder's hard against and perfectly centered at the chamber shoulder; the clearance is between the case head and bolt face where it's not important to have extremely tight tolerances.</p><p></p><p>Lets assume that we do have a bunch of cases with no more than 1/10,000ths inch spread in head to shoulder datum. And all their case heads are perfectly square with their long axis. Unless your bolt face is perfectly square with the chamber axis, as soon as you fire one case, its head will flatten out to pretty much full contact on your bolt face and take its angle. Full length sizing that case again will not square up its head. Rechambering it any other way except how it was indexed when first fired will cause a slight interference as the high points of both the case head and bolt face now start to align with each other. And that means the bolt will bind up when its closed on a live round without enough head clearance. Problem is, if the bolt doesn't close exactly back into the same place with the same pre-loading forces, the barrel's gonna whip differently for each shot as the bullet goes down the barrel and accuracy starts to suffer.</p></blockquote><p></p>
[QUOTE="Bart B, post: 635410, member: 5302"] Whopper Stopper, I've found it's not necessary to resize bottleneck cases to the nth degree of uniformity. Especially when brand spanking new ones will shoot 1/2 MOA at 600 yards and 3/4 MOA at 1000. They'll have a case headspace spread of about 2/1000ths inch; sometimes more. With cases not being perfectly uniform in metalurgy properties as well as wall thickness, the best us humans can hope for is a 1/1000th inch spread in sized bottleneck case headspace from head to shoulder datum. But 2/1000ths inch is good enough for what's called head clearance; the difference between chamber headspace and case headspace. Remember when a rimless bottleneck (as well as a properly sized belted bottleneck) round fires, the case shoulder's hard against and perfectly centered at the chamber shoulder; the clearance is between the case head and bolt face where it's not important to have extremely tight tolerances. Lets assume that we do have a bunch of cases with no more than 1/10,000ths inch spread in head to shoulder datum. And all their case heads are perfectly square with their long axis. Unless your bolt face is perfectly square with the chamber axis, as soon as you fire one case, its head will flatten out to pretty much full contact on your bolt face and take its angle. Full length sizing that case again will not square up its head. Rechambering it any other way except how it was indexed when first fired will cause a slight interference as the high points of both the case head and bolt face now start to align with each other. And that means the bolt will bind up when its closed on a live round without enough head clearance. Problem is, if the bolt doesn't close exactly back into the same place with the same pre-loading forces, the barrel's gonna whip differently for each shot as the bullet goes down the barrel and accuracy starts to suffer. [/QUOTE]
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measuring cases to .0001
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